The present invention relates generally to rapid-prototyping systems such as stereolithography and laser-sintering systems. The invention more particularly relates to the control of a laser scanning system during vector scanning.
Rapid prototyping and manufacturing (RP&M) is the name given to a field of technologies that can be used to form three-dimensional objects rapidly and automatically from computer data representing the objects. In general, rapid prototyping and manufacturing techniques build three-dimensional objects, layer-by-layer, from a working medium utilizing sliced data sets representing cross-sections of the object to be formed. Typically an object representation is initially provided by a Computer Aided Design (CAD) system. RP&M techniques are sometimes referred to as solid imaging and include stereolithography, ink jet printing as applied to solid imaging, and laser sintering.
A laser sintering apparatus dispenses a thin layer of heat-fusible powder, often a heat-fusible polymer powder, polymer coated metal, or ceramic, across a process chamber to create a bed of the powder. The laser sintering apparatus then applies thermal energy to melt those portions of the powder layer corresponding to a cross-section of the article being built in that powder layer. The article is formed within a mass of powder commonly referred to as the “part cake.” Lasers typically supply the thermal energy through modulation and precise directional control to a targeted area of the powder layer. Conventional selective laser sintering systems, such as the Vanguard system available from 3D Systems, Inc., use a carbon dioxide laser and position the laser beam using a scanning system having galvanometer-driven mirrors that deflect the laser beam.
There are two types of laser scanning commonly performed in rapid-prototyping systems: raster scanning and vector scanning. In raster scanning, the laser beam is scanned sequentially along a series of straight lines that are spaced apart and parallel to one another and that are relatively long (typically at least as long as the outside dimension of the part being fabricated in the scanning direction); thus, the laser beam has to move in only one direction along each scan line, and the scanning system typically is arranged such that the movement along each scan line is effected by movement of a single mirror. In vector scanning, the laser beam is scanned sequentially along a series of straight lines or vectors whose lengths can vary from very short (less than 1 mm) to relatively long, and whose orientations relative to one another can vary, such that in general it requires coordinated movement of two mirrors to scan a vector. The ending point of one vector often coincides with the starting point of the next vector. The present invention is concerned in particular with vector scanning, which has challenges that generally do not come into play in raster scanning.
Most of the commercially available vector scanners comprise two mirrors each mounted on the shaft of a galvanometer. In addition to galvanometers, brushless motors can be used to turn the shafts on which the mirrors are attached. A laser beam is reflected by the mirrors one after the other and then onto the working surface of a heat-fusible material in the rapid prototyping apparatus. Typically, the two mirrors are positioned above the working surface and the focused laser beam proceeds vertically downward onto the working surface. The galvanometric scanning mirrors are positioned so that when each of the shafts turns on its axis the two mirrors move the focused laser spot on the working surface. The two mirrors are arranged so that they move the spot in two orthogonal directions.
The laser scanners have also angular encoders attached to shafts. The angular encoders measure the change in angle of each shaft. Typically, special capacitors are used as angular encoders. When the angle of the shaft changes, the capacitance changes linearly with the angle. The encoder circuitry converts the values of capacitance values to voltages. Optical encoders have a similarly linear and monotonic encoder response curve.
The monotonic response voltage versus angle leads to problems in high-speed, high-resolution applications. In typical high-resolution scanner applications, 5 μRad angular resolution is expected. Typically, 5 μRad angular resolution may correspond to about 125 μV encoder signal. Thus, in order to achieve a high angular resolution the encoder electronics have to be able to separate voltage levels at about 100 μV accuracy. Moreover, this 100 μV accuracy needs to be achieved in a few tens of microseconds. It is very difficult to design a practical encoder circuitry that can resolve 100 μV in a few tens of microseconds. Thus, most commercial scanning systems are not able to measure the position of a scanning mirror to a high accuracy while the mirror is moving; accuracy is achieved only when the mirror is stationary.
In rapid prototyping applications, the scanning mirrors can vary in size and mass depending upon the particular application. In some cases, the mirrors can be relatively large and accordingly can have substantial mechanical inertia. The scanning system also has electrical inertia as a result of the inductance of the galvanometers or motors used for moving the mirrors. Consequently, it can take a considerable period of time to accelerate the scanning mirrors to their full speed. It has been found that ignoring the finite acceleration period of the mirrors can in some cases lead to unacceptably large following errors of the laser spot.
A further difficulty associated with rapid prototyping systems is the control of the laser power. It is generally desired to deliver a predetermined exposure (i.e., energy per unit area) pattern on the working surface. In the simplest case, the preferred exposure pattern is constant exposure inside the part and zero exposure outside the part. In many practical cases, however, the preferred exposure pattern is not a uniform pattern. For example, higher exposure at the border of exposed area will be often beneficial. At any rate, generally there is a predetermined optimum exposure for the various regions of a part being built, and it is desired to regulate the laser power so as to achieve the optimum exposure as closely as possible.
If the velocity of the laser spot is varying, a constant exposure along the vector to be scanned can be achieved by varying the laser power proportional to the speed of the laser spot. This is described in U.S. Pat. No. 6,085,122, the disclosure of which is incorporated herein by reference.
In order to control the exposure accurately, it is necessary to accurately control the scanning speed and the laser power. Typically, the power can be controlled by varying the excitation of the laser gain medium. For example, in the case of RF-excited CO2 lasers used in laser sintering, the RF power to the laser can be pulse-width modulated. Alternatively, the power can be controlled by letting the laser run at constant power and attaching a power modulator into the laser beam before the scanning mirrors. For example, an acousto-optic modulator (AOM) can be used for this purpose.
In practical applications, the power is measured in steady state conditions by applying different levels of modulation and measuring the laser power after the laser has reached the steady state condition. In this way, the functional relation between the modulation level and steady state laser power is established and stored in the laser power control system. This functional relation is called the power calibration curve. The power calibration curve is then used in order to achieve a constant exposure along the vector. At each control step, the laser spot velocity is determined and the power calibration curve is used to evaluate what level of power modulation is needed to make the laser power proportional to the laser spot velocity.
The approach described above assumes that the dynamic response of the laser power control can be ignored. Unfortunately, it is very important to consider the dynamic response of the laser when controlling the laser power by varying excitation of the laser medium. For example, it takes about 100 microseconds (“μs”) for a CO2 laser of the type used in laser sintering to reach its full power after it has been turned on, and it takes even longer to reach steady state power at low pulse-width modulation. Typical scanning mirror acceleration times are a few hundred microseconds to about a millisecond, as previously noted. If the dynamic response is not taken into account, the exposure will not be uniform all along the vectors. The beginnings of vectors will be underexposed and the ends will be overexposed.